1. Field of the Invention
The invention is directed to a power converter circuit for connecting a three-phase generator to an electrical network. Such circuits are used for instance in a wind power system. In that case, a dynamically varying output power of the generator is self-evident because of natural conditions, that is, due to varying wind speeds.
2. Description of the Related Art
The current generated in a generator must be fed into an existing electrical network in a way that is appropriate for the network in terms of voltage, frequency and phase. Typical magnitudes of voltages in such networks range from a few hundred to several tens of kilovolts and frequencies of 50 Hz or 60 Hz.
Prior art wind power systems which vary dynamically over time and include the following technologies.
German Patent Disclosure DE 101 14 075 A1 discloses a complex connection of a generator having a dynamic power output to a medium-voltage electrical network. This generator is connected to a bridge rectifier. The bridge rectifier generates a direct voltage of several kilovolts that is connected to a cascaded arrangement of inverter cells via a direct voltage connection. This extremely flexible power converter circuit can operate quite efficiently, because of the cascaded inverter cells and the upward converters integrated therewith, even when wind speeds vary greatly, causing the attendant output voltages of the generator to vary. Power converter circuits of this kind, however, suffer from a drawback, namely, that favorable, robust, asynchronous machines cannot be used with such circuits, since they require a magnetic rotating field for their excitation. Another disadvantage of such circuits is the relatively high complexity of circuitry, which, for example, precludes installing a circuit of this kind into existing wind power systems, for economic reasons at least.
U.S. Pat. No. 5,652,485 discloses a simple possibility for connecting a wind power system to an electrical network. According to this patent, a four-quadrant inverter is connected directly by its alternating voltage terminals between the output of the generator and the electrical network. Fuzzy logic provides suitable triggering of the four-quadrant inverter. It is understood here that the four-quadrant inverter must have a power capacity equivalent to the maximum power output by the generator. Advantageously, this power capacity is even slightly higher than that of the generator, to assure secure operation.
The simplest possible way of connecting a wind power system to an electrical network is the direct connection of the generator of the wind power system to the electrical network. Asynchronous generators are preferably used, because they are robust. However, such a simple circuit has several disadvantages. At low wind speeds, because the generator speed is kept constant, coupling to the electrical network is possible, if at all, only with highly restricted efficiency, because of the inadequate output power. Moreover, with this circuit arrangement, a reactive power compensation that the applicable energy supply business requires cannot be attained in every operating state.
The invention further describes a triggering method of a power converter circuit arrangement for reactive power compensation in the event of a network short circuit. In the event of a network short circuit, if the short circuit is located at a certain distance from the place to be observed, a residual voltage is generally preserved. If this residual voltage also has a defined value, which is dependent on the specifications of the network operator, for instance being 50% of the rated voltage, then an infeed point located there, such as a wind power system, must be capable of feeding reactive power into the electrical network. A wind power system of the construction described above, with direct coupling of the generator to the electrical network, cannot meet this precondition.